Activities in the Industrial Age have resulted in the deposit of high levels of many metals in certain sites, to the point that human life is seriously threatened. Metal-production activities, such as mining or smelting, as well as the ubiquitous use of metals, have created many sites where toxic metals have become concentrated in soils. Although the problem has been recognized for many years, and much effort has been expended on methods to remove the metals, existing techniques are cumbersome, expensive, and invasive.
In recent years, efforts have been made to utilize metal-accumulating plants to remove contaminating metals from sites (see, for example, Baker et al. New Scientist 1603:44, 1989; Chaney et al. in Land Treatment of Hazardous Wastes ed. by Parr et al., Noyes Data Corp: Park Ridge, pp 50-76, 1983). There are many advantages to using plants for remediation, including lower costs, generation of recyclable metal-rich plant residue, applicability to a range of toxic metals and radionuclides, minimal environmental disturbance, elimination of secondary air or water-borne wastes, and public acceptance.
Unfortunately, most of the known metal-accumulating plants are slow-growing, small and/or weedy plants that produce low biomass (see, for example, Baker et al. supra), so that even if the plants concentrate metals effectively, they cannot remove large amounts of metal from the soil. Furthermore, most plants that accumulate metals collect the metal in their roots rather than into their above-ground shoot portions. In fact, it is generally accepted that most plants do not accumulate significant levels of heavy metals into their shoots. Since metal accumulated into plant roots cannot be removed from the site until the plant roots themselves are harvested, standard phytoremediation protocols require that the roots be harvested, an expensive and complicated process.
There remains a profound need for improved methods of remediating metal-contaminated sites.